The invention relates to the field of aqueous emulsions and in particular to a process for the preparation of aqueous emulsions of fluorinated polymers.
Fluorinated copolymers are used in various applications, in particular in the hydrophobic and oleophobic treatment of various substrates, such as textiles, leather or paper. These fluorinated copolymers are generally prepared by copolymerization in an organic solvent or according to the aqueous emulsion polymerization technique. For reasons of environmental constraints, they are generally prepared by aqueous emulsion copolymerization. For hydrophobic and oleophobic coating applications, these copolymers are obtained from at least one perfluorinated acrylic monomer and at least one nonfluorinated monomer, generally an alkyl acrylate or methacrylate.
The copolymerization of perfluorinated monomers according to the conventional emulsion technique is a technically difficult problem. This is because the strong hydrophobicity of these monomers greatly decreases, on the one hand, the contribution of the initiation of the polymerization in the aqueous phase and, on the other hand, the diffusion of these monomers from the storage drops to the growing particles. As a result of these particular properties of perfluorinated acrylic monomers, the use of a water-soluble organic solvent is necessary when they are employed in producing an aqueous emulsion. This solvent has to be a good solvent for monomers and must not cause the polymer formed to precipitate, as is the case with certain alcohols.
The most widely used solvent is acetone (see, for example, Patents FR 1,532,053 or FR 2,202,515). However, safety and transportation constraints are prompting a large number of producers to market products without a flashpoint. It is obviously possible, if the surface-active formulation has been carefully selected, to distil off the solvent used in the copolymerization.
However, this decreases the productivity and results in an additional cost with regard to energy consumption, on the one hand, and confers poor stability over time on the emulsion, on the other hand.
The use of nonflammable solvents, such as ethylene glycol, propylene glycol and their derivatives, is disclosed in Patent FR 2,175,332 but these compounds generally exhibit a poor solvating power and must be used in combination with acetone. In some commercial fluorinated emulsions, acetone has been replaced by excellent solvents with a high flashpoint, such as N-methylpyrrolidone or xcex3-butyrolactone. The use of heavy diols, such as dipropylene glycol, pentanediol, hexanediol and tripropylene glycol, in producing fluorinated acrylic emulsions without a flashpoint is disclosed in Patent JP 60.40182. However, it turns out that such emulsions exhibit a number of drawbacks related to the low volatility of the solvents, such as:
difficult drying of textile and leather articles, which it is impossible to bring to high temperature, and/or
the lack of crosslinking of the copolymer on textiles, due to the incomplete drying of the solvent and to the presence of residual hydroxyl groups.
Whatever the solutions introduced by persons skilled in the art in terms of choice of solvent, it transpires that problems arise:
of volatile organic compounds, resulting in pollution, safety and transportation risks if the solvent cannot be distilled off,
of productivity and of an additional cost with regard to energy consumption, if the formulation comprises a solvent of low boiling point which is distilled off at the end of polymerization.
Furthermore, the conventional emulsion presents a problem of stability and of yield insofar as emulsion polymerization results in the formation of a high level of coagulum (1-8%), which coagulum has to be destroyed.